Core Project

SOUND - Interference Tube



Several of the extensions involve measurements of sound intensity in a standing wave of sound. For these investigations you will need a cardboard tube an inch or more in diameter, your Sound Level Meter, and the tone generator you built in the Core Project for a sound source.

Only minor changes in your equipment are required to assemble an experimental sound interference tube.
First, improve the Sound Level Meter by attaching the microphone to a piece of coaxial cable.


Every wire is potentially an antenna. Electromagnetic radiation can produce stray voltages in unshielded wires. The 60-Hz signal produced by the 120-volt service in most walls can be especially troublesome. Coaxial cable protects the small, unamplified signal from an electret microphone from stray electromagnetic interference.


  1. Strip about 1 cm of outer insulation from the end of the cable.
  2. Unwind the braided shielding using the point on scissors, a dissecting needle, or a similar tool.
  3. Starting about 1 cm from the end of the outer insulation, make a 1.5 cm slit parallel to the cable.
  4. Use scissors to tease the braided shielding through this slit. Twist the shielding wires together. Strip the inner insulation from the shielded wire.
  5. Solder the twisted strand of shielding wires to the negative pin of the electret microphone. Use a heat sink.
  6. Solder the shielded wire to the other pin. Use a heat sink.

C. SOLDERING THE COAXIAL CABLE TO SOLID CORE WIRES. At the other end, the shielding must be attached to a solid core wire so that it can be inserted into a breadboard. If the shielded wire is stranded, it must also be attached to solid core wire. This procedure is almost identical to the procedure just completed. Substitute short lengths of hook-up wire for the microphone pins. Make sure that the insulation on the ground (black) wire overlaps the insulation on the coaxial (gray) cable. Then both wires can be brought together and covered with electrical tape without creating a short circuit.

Attach the microphone and part of the coaxial cable to a metal rod so it can be moved inside the cardboard tube. Some suggestions are give below.

  1. Straighten the coat hanger or other wire that you are using for a rod.
  2. At one end of the rod, make a small bend to help push and pull the rod during the experiment.
  3. At the other end of the rod, bend a loop to hold the microphone -- not too tight (you will damage the microphone) or too loose (it will fall out during the experiment). In difficult cases, hold the microphone in place with small dabs of hot glue.
  4. Tape the coaxial cable to the rod for a length equal to the length of your cardboard tube. That's it. Your sound level meter is ready for measuring sound intensities inside the tube.

Adding sound to the interference tube is the easiest of all! Tape a speaker to one end of the cardboard tube. If there is a mismatch between the diameter of the speaker and tube, create a cardboard donut with inside and outside diameters that match your components. Then tape the speaker to the donut and the donut to the tube. Attach your Tone Generator to the speaker.


To change the effective length of the interference tube, create a movable, cardboard disk that closes the other end of your cardboard tube.

  1. Cut a cardboard disk that just fits your cardboard tube. It should be loose enough to easily move inside the cardboard tube.
  2. Cut a notch in the disk just large enough for the rod, coaxial cable, and tape to move through it easily. The microphone will not pass through this notch.
  3. The handle should be at least 10 cm longer than your your cardboard tube. Hot glue the handle to the center of the cardboard disk.


Below is a diagram of how the sound level meter, piston, and interference tube are combined to create the apparatus for investigating standing waves. You are ready to begin.

TESTING the apparatus

1. Move the microphone and the plunger as close to the speaker as practical. Keeping the microphone and the plunger together, pull them down the length of the tube. Note the change in the sound intensity (measured in volts on your multimeter) as you do this. If all is well, the voltages should rise and fall as the microphone moves down the tube. Stop testing when the plunger is close to the far end of the cardboard tube.

2. Keeping the plunger and the microphone close together, return to the first local maximum or minimum in voltage. Keep the plunger in this position for the remainder of the experiment. Measure the sound intensity at various positions inside the tube. Look at an animation of the data gathering.

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